The field of the present invention relates to distributed optical devices. In particular, apparatus and methods are described herein for employing individual element amplitude and phase control in distributed optical structures.
Distributed optical structures in one-, two-, or three-dimensional geometries offer powerful optical functionality and enable entirely new families of devices for use in a variety of areas including optical communications, spectral sensing, optical waveform coding, optical waveform processing, and optical waveform recognition. It is important in the design of distributed optical structures to have means to control the amplitude and phase of the electromagnetic field diffracted by individual diffractive elements within the overall distributed structure. This invention relates to approaches for fabricating diffractive elements that provide flexible control over diffractive amplitude and phase.
A distributed optical structure typically includes a large number of individual diffractive elements. Each individual diffractive element may scatter (and/or reflect and/or diffract) only a small portion of the total light incident on the distributed structure. This may be because the individual diffractive elements subtend only a small fraction of available solid angle of the incident optical field in the interaction region, and/or because individual diffractive elements have a small reflection, diffraction, or scattering coefficient. Distributed optical structures in two or three dimensions can also be described as volume holograms since they have the capability to transform the spatial and spectral properties of input beams to desired forms.
There are many reasons why it is important to have control over the amplitude and/or phase of the portions of the field scattered by individual diffractive elements. For example, a distributed optical structure can act as a general spectral filter supporting a broad range of transfer functions. In the weak-reflection approximation, the spectral transfer function of a structure is approximately proportional to the spatial Fourier transform of the structure""s complex-valued scattering coefficientxe2x80x94as determined by the amplitude and phase of the field scattered by individual diffractive elements (See T. W. Mossberg, Optics Letters 26, 414 (2001) and the provisional applications cited hereinabove). In order to produce a general spectral transfer function, it is useful to control the amplitude and phase of each constituent diffractive element. Application of the present invention provides for such control. Also, when multiple distributed structures are overlaid in the same spatial region, system linearity can only be maintained by ensuring that the diffractive strength of overlaid diffractive elements is the sum of the individual diffractive element strengths. When diffractive elements are lithographically scribed, overlaid structures will not typically produce a summed response. The approaches of the present invention provide means for modifying overlaid diffractive elements (formed by lithographic and/or other suitable means) so that each element negligibly affects another""s transfer function.
An optical apparatus according to the present invention comprises an optical element provided with at least one set of at least two diffractive elements. Each diffractive element diffracts a corresponding diffracted component of an incident optical field with a corresponding diffractive element transfer function. Collectively, the diffractive elements provide an overall transfer function between an entrance optical port and an exit optical port (which may be defined structurally and/or functionally). Each diffractive element is spatially defined by a corresponding diffractive element contour and includes at least one diffracting region of the corresponding contour modified in some way so as to diffract, reflect, and/or scatter a portion of an incident optical field. The modification of the contour typically involves a differential between some optical property of the diffracting region relative to the corresponding average optical property of the optical element (effective index, bulk index, surface profile, and so forth). At least one of: i) the overall transfer function; and ii) at least one corresponding diffractive element transfer function, is determined at least in part by at least one of: a) a less-than-unity fill factor for the corresponding contour; b) a non-uniform distribution of multiple diffracting regions of the corresponding contour; c) variation of a spatial profile of the optical property along the at least one diffracting region of the corresponding contour; d) variation of a spatial profile of the optical property among multiple diffracting regions of the corresponding contour; and e) variation of the spatial profile of the optical property of the at least one diffracting region among the elements in the diffractive element set.
The optical element may be a planar or channel waveguide, with optical field propagation substantially confined in at least one transverse dimension. In a waveguide, the diffracting segments are curvilinear segments having some alteration of an optical property relative to the waveguide. The optical element may enable three-dimensional propagation of optical fields therein, with the diffracting segments being surface areal segments of surface contours within the volume of the optical element. The optical element may be a diffraction grating, the diffracting segments being segments of the grating lines groove contours that are formed on the grating. These various distributed optical devices may define one or more ports, and may provide one or more spatial/spectral transfer functions between the one or more ports.
Various objects and advantages of the present invention may become apparent upon referring to the preferred and alternative embodiments of the present invention as illustrated in the drawings and described in the following written description and/or claims.